This invention relates to an over stress or overload protective circuit for use with output transistors in direct current power supplies of the switching or power flyback type, and more particularly to improved circuit means to protect power drive transistors of the power supply from excessive stress or internal power dissipation and consequent damage due to over stress during overload.
The application of overloads on drive or output transistors in a flyback or switching power supply necessitates automatic protective circuits which are fast acting to protect the output transistors and related equipment from damage due to over stress and other excessive current effects. The use of sensing circuits in an attempt to limit excessive power supply transistor stress of the main drive or output transistors during overload of a flyback power supply has been difficult to achieve because the load current circuit is out of phase with current applied to the driver for the main output transistors and output sensing occurs only after heavy output transistor current has resulted in damage. Another disadvantage of the usual sensing circuitry is the main power or switching transistors usually draw excess current prior to complete shut down in response to excess load resulting in interruption of output power as well as damage to the power transistors before the output sensing circuitry can stop the current build up. For example, when the power supply load becomes too heavy, the main power transistors draw heavy current and sensing of this current in the output circuit occurs too late to prevent damage due to stress.
Accordingly, it is an object of the invention to provide a novel circuit to protect a regulated transistor power supply from loads connected thereto from causing damage due to overload, particularly to the main power transistors.
Another object of the invention is to provide an improved protective circuit which avoids one or more of the disadvantages or limitations of the prior art arrangements.
A further object of the invention is to protect main power transistors and associated components in a direct current power supply, such as of the flyback type, from excessive stress or internal power dissipation due to current overload during operation of the power supply.